During the past several years, the popularity and viability of fuel cells for producing large and small amounts of electricity has increased significantly. Fuel cells conduct an electrochemical reaction with chemicals such as hydrogen and oxygen to produce electricity and heat. Fuel cells are similar to batteries, but they can be “recharged” while providing power, and are much cooler and cleaner than devices that combust hydrocarbons.
Fuel cells provide a DC (direct current) voltage that may be used to power motors, lights, computers, or any number of electrical appliances. There are several different types of fuel cells, each using a different chemistry. Fuel cells are usually classified by the type of electrolyte used. The fuel cell types are generally categorized into one of five groups: proton exchange membrane (PEM) fuel cells, alkaline fuel cells (AFC), phosphoric-acid fuel cells (PAFC), solid oxide fuel cells (SOFC), and molten carbonate fuel cells (MCFC).
Each of the fuel cells mentioned above uses oxygen and hydrogen to produce electricity. The oxygen for a fuel cell is usually supplied by the ambient air. In fact, for the PEM fuel cell, ordinary air may be pumped into the cathode. However, hydrogen is not as readily available as oxygen. Hydrogen is difficult to generate, store and distribute.
One common method for producing hydrogen for fuel cells is the use of a reformer. A reformer produces hydrogen from hydrocarbons or alcohol fuels. The hydrogen can then be fed to the fuel cell. However, if the hydrocarbon fuel is gasoline or some of the other common hydrocarbons, undesirable byproducts are produced, such as SOx, NOx and others. These byproducts are not only pollutants, but can damage the reformer. Sulfur, in particular, must be removed from the reformer or may damage the electrode catalyst. Additionally, reformers usually operate at high temperatures and consume significant energy.
Alternatively hydrogen can be generated from a precursor at ambient temperature using a catalyst. However, such chemical reactions for producing hydrogen may require a pump to move the precursor, a hydrogen-bearing chemical mixture, into a reaction chamber filled with a catalytic agent. As soon as the chemical mixture is exposed to a catalyst, the reaction rate is accelerated. Thus, the chemical mixture and catalyst must be separated until hydrogen production is to start. Consequently, a pump is needed to selectively move the chemical mixture from storage to the reaction chamber.
Further, for portable fuel cell applications, it is difficult to miniaturize the fuel cell and hydrogen-producing system, and still produce hydrogen on demand. Once a chemical reaction in the presence of a catalyst has begun, the reaction is difficult to stop and/or restart. The electrical demands of portable electronics may vary widely, therefore a fuel cell providing power to portable electronics must be equipped to efficiently provide varying amounts of hydrogen on-demand to produce the electricity needed.
One solution to produce hydrogen on-demand is to use micro-pumps to deliver a certain amount sodium borohydride (hydrogen-bearing solution) to a catalyst bed. However, a by-product of the sodium borohydride, sodium metaborate, tends to absorb water and gel when allowed to cool. This hinders access to the catalyst and renders the water needed for the reaction unavailable.
Another solution is to heat the sodium borohydride, which increases the rate of hydrogen production. However, using heat to increase hydrogen production on-demand results in higher parasitic losses.